1. Field of the Invention
The present invention relates generally to a shift control system for an automatic power transmission of an automotive vehicle. More specifically, the invention relates to a shift control system which selects an optimal transmission speed ratio shifting pattern depending upon vehicle driving conditions. Further particularly, the invention relates to a shifting pattern selection during up- and down-hill driving for selecting an optimal speed ratio shifting pattern for enhanced shift feeling.
2. Description of the Background Art
Some of modern automatic power transmissions are associated with electric or electronic control systems which detect vehicle driving conditions and which select an optimal transmission speed ratio. In such shift control systems, a plurality of speed ratio shifting patterns are set so that optimal one of the patterns is selected depending upon the vehicle driving conditions. Typically, the shift patterns are determined experimentarily and set in relation to typical driving conditions. Therefore, under a vehicular driving condition coincidence with one of the typical driving conditions or close thereto, the transmission speed ratio selection results in providing optimal shifting performance. In contrast to this, when a vehicle driving condition is far different from any one of the typical driving conditions, such as up- or down-hill driving, when the passenger load is substantially heavy, under cold engine condition and so forth, the speed ratio selection fails to provide optimal shifting performance.
For instance, since the transmission speed ratio shift pattern is generally set in terms of a vehicle speed and a throttle valve open degree. In the hill climbing, the vehicle speed tends to vary according to the gradient of the road. Therefore, there has been experienced a so-called busy shifting between a higher speed ratio and a lower speed ratio. Such busy shifting may cause degradation of the riding comfort of the vehicle and vehicular drive feeling. For avoiding this, Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-180153 discloses a transmission speed ratio shift control technology which includes detection of the hill climbing condition of the vehicle on the basis of the engine load and the vehicle speed. In the disclosed technology, the vehicle speed is monitored in relation to the engine load, e.g. the throttle valve open angle, for making judgement of hill climbing when the vehicle speed is lowered at constant engine load for selecting shift pattern set for hill climbing. Alternatively, Japanese Patent First Publication (Tokkai) Showa 62-165052 discloses a shift control system which monitors a frequency of shifting and thus detects busy shifting based on the frequency of occurrence of shifting to vary shifting pattern for avoiding busy shifting.
On the other hand, Japanese Patent First Publication (Tokkai) Showa 63-167161 discloses passenger load dependent shift pattern selection, in which number of passenger boarding on a vehicle is counted and shift pattern is selected based on the number of boarded passengers.
These prior proposed shift control systems are effective at specific vehicle driving conditions. Therefore, such prior proposed system cannot provide optimal selection of the transmission speed ratio in the practical driving condition.
As can be appreciated, an optimal or ideal speed ratio selection should establish a transmission speed ratio at which enough but not excessive driving torque is applied to the driving wheels, depending upon the running resistance exerted on the vehicle. Therefore, in order to appropriately select the transmission speed ratio, it is important to precisely detect the running resistance on the vehicle. In case of a manual transmission, running resistance is measured by a driver so that he may select an optimal gear position based on the measured running resistance. Therefore, level of skill of selection of the gear position in the manual transmission is determined depending upon the precision level of judgement of the driving condition. Therefore, the ideal shift control system for the automatic power transmission has to have capability of precise judgement of the vehicle driving condition equivalent to that of the qualified driver.
In this sense, the prior proposed shift control systems are not at all complete either in judgement of the vehicle driving condition or in selection of the speed ratio.